Bonding to Existing Concrete

posted by Bob

Fact: Fresh wet concrete does not normally bond well to existing dry concrete. Do you remember elementary school where one of the subjects on which you were graded was “plays well with others”? Concrete would have gotten an F. There is nothing in basic portland cement that will act as a bonding agent. Portland cement concrete works well in mass and provides great compressive strength but not bond.

Concrete is marvelous stuff but in time it will deteriorate. When it does, you either have to patch it or replace it. Assuming that it is structurally sound the least expensive alternative is to patch it. However patching it requires some attention to detail or your patch will not last. So that you don’t waste too much time or money, we should probably discuss what “structurally sound” means. If your sidewalk has either heaved or dropped at almost every joint, repairing it will not provide a long-term solution. The slabs are likely still moving. If your slab has so much sand and gravel on the surface that despite sweeping and sweeping and squirting and squirting it just keeps coming back, don’t waste your time on repairs. If you have multiple cracks that run so deep that they appear to run through the slab, a repair would only be temporary. The solution to all of these problems involves a jackhammer and bags of one of the Sakrete concretes.

Since this discussion is on the best way to bond concrete, we will assume that your slab is good.

There are a variety of Sakrete concrete repair products available to fix concrete that has begun to deteriorate. However, without good surface preparation, none of them are going to perform satisfactorily. All loose sand, gravel, dirt, leaves etc. must be removed. This can typically be done with a garden hose and a good nozzle. Tough areas may require a pressure washer or mechanical abrasion. The two toughest areas to cover are those with oil and tree sap. Both of these will work their way down into the concrete. Simply washing the surface isn’t sufficient. If the stains do not run too deep, you can chip away the concrete using a hammer and chisel. Don’t forget the goggles (not just glasses) as this process will throw concrete all over the place. Also, keep your thumb out of the way. If the spots are too large or too deep for this to be practical, you may need a sealer to cover the stains before patching.

There are two basic methods for bonding a portland cement based product to existing concrete; 1) chemically and 2) mechanically.

Let’s discuss the mechanical approach first since it is really used in both approaches. The most effective way to ensure a really good bond is with a scratch coat. This is simply a very wet coat made up by mixing the repair product with water. Mix up a small amount of the repair material to a soupy consistency. You don’t need to measure the water-just turn the stuff into slop. Then, using a gloved hand or a rag, smear the material onto the area to be patched. Just think finger painting from kindergarten. The technique is about the same. Apply pressure to ensure that as much as possible is shoved into the nocks and crannies. You only need a thin coat. It is not necessary for this scratch coat to dry. By the time you get the repair material mixed it will be ready. Then mix up additional repair material to the proper consistency and apply over this thin scratch coat.

The chemical approach involved mixing up a liquid bonding agent that helps bond new concrete products to old. Products like Sakrete Top'n Bond and Sakrete Flo-Coat already contain polymers that greatly improve the bond of portland cement and should NEVER be used with a liquid bonding agent. I know in America bigger is better but it’s just not so with these products. Other products like Sakrete Sand Mix and Sakrete Fast Set Cement Patcher benefit from the use of a liquid chemical bonding agent such as Sakrete Bonder & Fortifier. When using a liquid bonding agent, paint the bonder onto the existing concrete and allow it to dry until it is tacky. This usually takes only a few minutes. Then apply the repair material. Just as in the process described above, after the bonder has become tacky apply a scratch coat and then apply the repair material. The most effective way to ensure that the bonding agent gets into the existing concrete is to apply it directly using a brush or rag. It can be sprayed if you happen to have a sprayer. Although the directions say that you can use it as part of the mix water, direct application works better.

If you are doing a large area and a scratch coat isn’t practical you will need to spray the surface with water before you apply the repair material. On a warm day, the existing concrete surface will be hot enough to suck the water out of the repair material. In addition, some concretes are quite porous and will rob water from your repair material. If too much water is lost into the old concrete there will not be enough water to hydrate all of the cement particles and a lower strength material will be the result. Concrete simply will not bond to all substances. Paint, oil, glue from old flooring tiles are just a few. You must mechanically remove these materials if you want the job to last.

Once the job is complete, you can do a quick check to see if the bond was successful. Wait at least 24 hours and then tap “gently” on the patch using a hammer or some other dull object and listen for a hollow echoing sound. If you just get a dull thud then the material has bonded well. If you get a hollow sound, the material has not bonded and will crack in time. Which means it is back to the beginning of today’s topic. Here is hoping your concrete work comes across as a dull thud (not like some of my party guests) rather than a hollow endeavor.

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772 USER COMMENTS

Clint, Clean the concrete slab porch with a power washer removing any dirt, paints sealer or any loose and foreign material. Sakrete Bonder & Fortifier will aid in adhering to the subsurface.
- Chris-Techncal Service
Wednesday, May 24, 2017 at 4:11 PM

Mark, you could use Sakrete Bonder & Fortifier on the surface, allow it to dry, then add 2" of the Sakrete Sand Mix Topping and Bedding Mix. You could also use the Top'n Bond in 1/2" layers, wait at least 1 hour between layers. Be sure to have all the layers done within 24 hours. Your biggest concern will be the prep work. So you are going to have to really clean the concrete area with a pressure washer to remove any loose dirt, debris, salts paints and or sealers. Without a proper prep job the product will not stick.
- Lee-Technical Service
Monday, May 22, 2017 at 3:30 PM

Kim, sorry but we do not have anything in our product line that would fit with your application. Try a local commercial flooring store. There are commercial products that will work on your application.
- Lee-Technical Service
Monday, May 22, 2017 at 10:36 AM

I have a concrete slab porch that I would like to raise 3" to match the height of the interior floor. What is the best method to pour over the existing slab?
- Clint
Saturday, May 20, 2017 at 8:59 AM

hi i have a 6'x6' patio cement slab and i had slate on top i just removed the slate last week and need to bring the slab up 2" can i use top& bond in 1/2" layers to get to my 2" if not what can i do? thanks
- mark
Friday, May 19, 2017 at 8:25 AM

I HAVE AN EXISTING 12" DEPRESSED SLAB THAT NEEDS TO BE INFILLED WITH LIGHTWEIGHT CONCRETE TO MATCH THE HEIGHT OF EXISTING ADJACENT FLOOR (12" THICK). HOW SHALL I PREP THE CONCRETE AND WHAT SHALL I USE TO BOND THE EXISTING TO THE NEW?
- KIM
Wednesday, May 17, 2017 at 3:37 PM

Junjun, You can use Top'n Bond Concrete Patcher for that project. This material can be placed from 1/2" down to a featheredge. So you will need to do 2 layers. Allow the first 1/2"layer to dry for 1 hour and then come back with a second 1/2" layer. Make sure to properly prep your surface. There can be no paints, sealers, dust, debris, and or oil present on the surface.
- Lee-Technical Service
Wednesday, May 17, 2017 at 10:52 AM

Rodgers, you would use Sakrete Bonder & Fortifier as a bonding agent for bonding old concrete to new concrete. As far as a resurfacing chemical, we do not have one. You can keep with an epoxy coating manufacturer for that type of product.
- Lee-Technical Service
Wednesday, May 17, 2017 at 10:42 AM

there is a step, an inch from the garage floor to driveway. How could I raise the driveway to 1" thickness by 12" W X 24ft. to level with garage floor and How?
- junjun tiongco
Wednesday, May 17, 2017 at 12:25 AM

I need to raise the concrete to about 100mm and there is already a concrete on the floor. What chemical can I use to bond the new and old concrete together? Secondly, I did concrete on a wash bay and the top area is pilling out, what chemical can i use to resurface without adding concrete and to avoid such to happen again in future? Regards Rodgers
- Rodgers
Friday, May 12, 2017 at 4:36 AM

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